This invention relates generally to imaging systems and, more specifically, to the correction of phase aberrations in imaging systems employing large optical elements. Differences in phase arise not only from differences in construction and geometrical relationships, but also from factors that may vary with time. In some imaging applications, such as those involving optical surveillance or the use of relay mirrors, there is a requirement to employ very large optical elements, usually mirrors. In many instances, these large optical elements need to be of very high quality, having an accuracy in the order one twentieth of a wavelength (.lambda./20). Light of wavelength 248 nm (nanometers), for example, requires tolerances of around 1.2.times.10.sup.-6 cm to achieve phase coherence to within one twentieth of a wavelength. Typically, the technology simply does not exist to fabricate the optical elements to this high degree of accuracy. Often when the technology is available, the costs of manufacture are prohibitively high.
Accordingly, there is a critical need for a technique that employs optical elements of lower quality but still produces the desired phase quality.
One well known approach to the correction of phase aberrations is to employ principles of adaptive optics. Basically, this approach employs one or more deformable mirrors, which are large reflecting surfaces made up of separately movable elements, each driven by a transducer, such as a piezoelectric device. The character of the optical wavefront emanating from such a mirror has to be sensed with a complex and highly sensitive interferometer, and then the composite wavefront has to be converted to electrical form, stored in an electronic memory, and manipulated mathematically to determine the magnitude of elemental corrections that have to be made in the deformable mirror.
The adaptive optics approach is inherently slow, because of its reliance on mechanical elements to effect phase compensation. The approach is also subject to errors due to intermirror optical path length differences, called "piston errors." Compensation of these errors has required the use of very complex arrangements of interferometry and adaptive optical components. The approach becomes even less practical as the size of the desired beam aperture increases. For large apertures, in the order of ten meters in diameter, deformable mirrors having as many as 10,000 elements may be required. Since each element is of finite size, the array has limited resolution and ability to correct wavefront distortions. Moreover, the cost and reliability of deformable mirrors of this magnitude have posed serious limitations to the development of a practical system using adaptive optics.
By way of further background, the invention also relates to the field of phase conjugate optics. It has been recognized for some time that phase conjugation of light waves can be used to remove phase aberrations caused by the passage of a light beam through a distorting or phase-aberrating medium.
There is extensive literature on the subject of phase conjugate optics and the use of phase conjugation for the compensation of phase aberrations. A summary of the history and principles of phase conjugate optics is provided in a paper entitled "Phase Conjugate Optics and Real-Time Holography," by Amnon Yariv, IEEE Journal of Quantum Electronics, Vol. QE-14, No. 9, Sept., 1978, pp. 650-60.
Simply stated, a phase conjugation cell functions as a reflector with a special and useful property. When an incident light wave is focused into the cell, the reflected wave that emerges is the complex conjugate of the incident wave. The practical consequence of the phase conjugation is that the retro-reflected wave is as if it were "time-reversed" with respect to the incident wave. For example, if an incident wave, after passing through a distorting medium, has a bulge in its wavefront, representing a phase-lagging condition at a particular region of the front, this will be reflected as an opposite bulge, i.e. a phase lead, in the same region of the reflected wavefront. If the reflected wavefront then traverses the same distorting medium that caused the original bulge in the incident wavefront, the reflected wave will emerge from the distorting medium as an undistorted wave.
In spite of the existence of a large body of theoretical knowledge concerning the principles of phase conjugate optics, prior to the present invention these principles have not been applied to the problem with which the invention is concerned. It will be appreciated from the foregoing that there is still a critical need for an alternative approach to the problem of providing phase-corrected beams in systems employing large optical elements, without building extremely high accuracy into the optical elements themselves. The present invention is directed to this end.